Introduction: Spinal stereotactic radiosurgery (SRS) has emerged as an attractive method to deliver high doses of radiation to oligometastatic spinal tumors of radioresistant histology. As this is a palliative therapy, attention to potential radiation toxicities is paramount when counseling patients. The objective of this study is to report a previously undescribed complication, radiation induced myositis.
Methods: Eleven patients with radiographic evidence of myositis following spinal radiosurgery underwent a retrospective chart review. Clinical and pathologic results were collected, including radiation dose, equivalent dose in 2 Gy fractions (EQD2), biologically effective dose (BED), and volume of muscle treated. Treatment toxicities were classified according to the Common Terminology Criteria for Adverse Events (CTCAE) version 4.03.
Results: Eleven patients were reviewed. Eight patients underwent single fraction 24 Gy treatment and three underwent 9 Gy x 3 fractions. The rate of myositis for patients undergoing single fraction treatment was 1.4% and was 0.9% for patients undergoing 3 fraction treatment. The median of the mean dose to muscle with myositis was 17.5 Gy. The median EQD2 was 55.1 Gy and the median BED was 82.7 Gy. Median time to development of clinical symptoms was 1.4 months while the median time to imaging evidence was 4.7 months. Five patients received anti-VEGF therapy. Two patients (18.2%) had CTCAE grade 3 complications.
Conclusions: Radiation myositis following spinal radiosurgery is a rare but important complication. Single fraction treatment schedules and anti-VEGF therapy may be associated with increased risk of myositis. We present the first series of radiation myositis in patients who underwent spinal radiosurgery for metastatic spinal disease.
Patient Care: Identifying and understanding radiation induced myositis is important when treating patients with radiosurgery. It is important to consider on the differential for radiographic changes following radiation, in addition to recurrent tumor. Given this is the first time this finding has been described, it adds to the literature and to clinician knowledge. In some cases, biopsy, and its risks, may be avoided.
Learning Objectives: 1. Describe the clinical and radiographicl findings of radiation myositis
2. Define the incidence and risk factors of radiation induced myositis and time course
3. Discuss treatment options for patients with radiation induced myositis.